COLUMBUS, Ohio -- Researchers at Ohio State University
have found a way to use hydrated lime to reduce the amount of
selenium emitted by fossil-fuel combusters and incinerators.

Selenium is one of the most volatile and toxic byproducts produced
by coal-fired power plants. An essential trace element for humans
and other animals, large quantities of selenium can cause infertility
and damage to the liver, kidneys and lungs.

About 2,500 tons of selenium are discharged into the atmosphere
each year in the United States alone. Regulations being proposed
by the Environmental Protection Agency will seek to control emissions
of selenium and other metals found in fly ash and flue gas, said
Liang-Shih Fan, professor and chair of chemical engineering at
Ohio State.

Fan and Suhas Mahuli, a post-doctoral researcher in chemical engineering,
have studied ways to capture selenium from flue gas before it
escapes into the atmosphere. The researchers found that hydrated
lime, which has been shown effective in controllingsulfur dioxide
emissions in flue gas, can also be used to reduce selenium.

The fine, powdery lime is injected into the plant's boiler chambers
through nozzles or injectors that can be retro-fitted to existing
plants. The lime particles capture selenium and sulfur dioxide.
As the flue gas moves upward, an electrostatic precipitator removes
the particles by electrical charging.

The method, called dry sorbent injection, has been studied for
several years as a way to remove sulfur dioxide from flue gas.
But this is the first time studies have shown that hydrated lime
chemically removes selenium by means of an irreversible chemical
reaction.

"The hydrated lime -- which is calcium-based -- forms with
the selenium to create calcium selenite, the first time this reaction
has been documented," Fan said. "The compound is then
captured by the dry sorbent and can be disposed of with the fly
ash."

The researchers found that the hydrated lime was most effective
at 400 to 600 degrees Celsius, and able to "capture"
up to five percent of its weight in selenium. At higher temperatures,
calcium selenite is thermally unstable, which reduces the efficiency
of selenium capture.

The scientists compensated for this by injecting the dry sorbent
further away from the firing point in a power plant, where temperatures
can exceed 1,000 degrees Celsius.

"At 400 to 600 degrees Celsius, the rate of capture should
be more than sufficient to effectively remove the selenium from
the gas environment," Mahuli said.

More work is needed to discover the best way to stabilize and
dispose of fly ash and the lime sorbent, which will contain the
selenium, Fan said. There are currently no regulations regarding
selenium disposal. The researchers are also trying to find out
how much it would cost to retro-fit a plant for this technology.

"It's unclear just how strict the EPA regulations on selenium
are going to be," Fan said. "Some plants may find it
easier to install a very expensive wet scrubber to remove all
of the harmful byproducts from flue gas and fly ash."

But dry sorbent injection technology could be more cost efficient
for the removal of selenium, and possibly arsenic. Another advantage
to the technology is the abundance of the raw material limestone,
especially in the Midwest, Fan said.

The study was co-authored by Rajeev Agnihotri, a doctoral student
in chemical engineering, and Abhijit Ghosh-Dastidar, a former
post-doctoral researcher, both from OhioState. The work, which
was published in a recent issue of the journal Environmental Science
and Technology, was supported by the OhioCoal Development Office,
a collaborative effort between OhioState and four other Ohio universities.